In the manufacture of tissue products such as bath tissue, a wide variety of product characteristics must be given attention in order to provide a final product with the appropriate blend of attributes suitable for the product's intended purposes. Improving the softness of tissues is a continuing objective in tissue manufacture, especially for premium products. Softness, however, is a perceived property of tissues comprising many factors including thickness, smoothness, and fuzziness.
To date, in many applications two-ply tissues generally have achieved improved softness over one-ply tissues. However, in terms of manufacturing economy, multiple-ply products are typically more expensive to produce than single-ply products. Thus, a need exists for a single-ply tissue product with high bulk and softness while retaining smoothness and strength.
Traditionally, tissue products have been made using a wet-pressing process in which a significant amount of water is removed from a wet-laid web by pressing the web prior to final drying. In one embodiment, for instance, while supported by an absorbent papermaking felt, the web is squeezed between the felt and the surface of a rotating heated cylinder (Yankee dryer) using a pressure roll as the web is transferred to the surface of the Yankee dryer for final drying. The dried web is thereafter dislodged from the Yankee dryer with a doctor blade (creping), which serves to partially debond the dried web by breaking many of the bonds previously formed during the wet-pressing stages of the process. Creping generally improves the softness of the web, albeit at the expense of a loss in strength.
Recently, throughdrying has increased in popularity as a means of drying tissue webs. Throughdrying provides a relatively noncompressive method of removing water from the web by passing hot air through the web until it is dry. More specifically, a wet-laid web is transferred from the forming fabric to a coarse, highly permeable throughdrying fabric and retained on the throughdrying fabric until it is at least almost completely dry. The resulting dried web is softer and bulkier than a wet-pressed sheet because fewer papermaking bonds are formed and because the web is less dense. Squeezing water from the wet web is eliminated, although subsequent transfer of the web to a Yankee dryer for creping is still often used to final dry and/or soften the resulting tissue.
Even more recently, significant advances have been made in high bulk sheets as disclosed in U.S. Pat. Nos. 5,607,551; 5,772,845; 5,656,132; 5,932,068; and 6,171,442, which are all incorporated herein by reference. These patents disclose soft throughdried tissues made without the use of a Yankee dryer. The typical Yankee functions of building machine direction and cross-machine direction stretch are replaced by a wet-end rush transfer and the throughdrying fabric design, respectively.
When the single ply tissue products, however, are formed into a rolled product, the base sheets tend to lose a noticeable amount of bulk due to the compressive forces that are exerted on the base web during winding and converting. As such, a need currently exists for a process for producing a single ply tissue product that has both softness and bulk when spirally wound into a roll. More particularly, a need exists for a spirally wound product that can maintain a significant amount of roll bulk and sheet softness even when the product is wound under tension to produce a roll having consumer desired firmness.
Definitions
A tissue product as described in this invention is meant to include paper products made from base webs such as bath tissues, facial tissues, paper towels, industrial wipers, foodservice wipers, napkins, medical pads, and other similar products.
Roll Bulk is the volume of paper divided by its mass on the wound roll. Roll Bulk is calculated by multiplying pi (3.142) by the quantity obtained by calculating the difference of the roll diameter squared in cm squared (cm2) and the outer core diameter squared in cm squared (cm2) divided by 4 multiplied by the sheet length in cm multiplied by the sheet count multiplied by the bone dry Basis Weight of the sheet in grams (g) divided by cm squared (cm2).
Roll Bulk in cc/g=3.142×(Roll Diameter squared in cm2−outer Core Diameter squared in cm2)/(4×Sheet length in cm×g/cm2) or Roll Bulk in cc/g=0.785×(Roll Diameter squared in cm2−outer Core Diameter squared in cm2)/(Sheet length in cm×g/cm2).
For various rolled products of this invention, the bulk of the sheet on the roll can be about 11.5 cubic centimeters per gram or greater, preferably about 12 cubic centimeters per gram or greater, more preferably about 13 centimeters per gram or greater, and even more preferably about 14 centimeters per gram or greater.
Geometric mean tensile strength (GMT) is the square root of the product of the machine direction tensile strength and the cross-machine direction tensile strength of the web. Geometric tensile strengths are measured using a MTS Synergy tensile tester using a 3 inches sample width, a jaw span of 2 inches, and a crosshead speed of 10 inches per minute after maintaining the sample under TAPPI conditions for 4 hours before testing. A 50 Newton maximum load cell is utilized in the tensile test instrument.
The Kershaw Test is a test used for determining roll firmness. The Kershaw Test is described in detail in U.S. Pat. No. 6,077,590 to Archer, et al., which is incorporated herein by reference. FIG. 4 illustrates the apparatus used for determining roll firmness. The apparatus is available from Kershaw Instrumentation, Inc., Swedesboro, N.J., and is known as a Model RDT-2002 Roll Density Tester. Shown is a towel or bath tissue roll 200 being measured, which is supported on a spindle 202. When the test begins a traverse table 204 begins to move toward the roll. Mounted to the traverse table is a sensing probe 206. The motion of the traverse table causes the sensing probe to make contact with the towel or bath tissue roll. The instant the sensing probe contacts the roll, the force exerted on the load cell will exceed the low set point of 6 grams and the displacement display will be zeroed and begin indicating the penetration of the probe. When the force exerted on the sensing probe exceeds the high set point of 687 grams, the value is recorded. After the value is recorded, the traverse table will stop and return to the starting position. The displacement display indicates the displacement/penetration in millimeters. The tester will record this reading. Next the tester will rotate the tissue or towel roll 90 degrees on the spindle and repeat the test. The roll firmness value is the average of the two readings. The test needs to be performed in a controlled environment of 73.4±1.8 degrees F. and 50±2% relative humidity. The rolls to be tested need to be introduced to this environment at least 4 hours before testing.
The Fuzz-On-Edge Test is an image analysis test that determines softness. The image analysis data are taken from two glass plates made into one fixture. Each plate has a sample folded over the edge with the sample folded in the CD direction and placed over the glass plate. The edge is beveled to 1/16″ thickness.
Referring to FIG. 5, one embodiment of a fixture that can be used in conducting the fuzz-on-edge test is shown. As illustrated, the fixture includes a first glass plate 300 and a second glass plate 302. Each of the glass plates have a thickness of ¼ inch. Further, glass plate 300 includes a beveled edge 304 and glass plate 302 includes a beveled edge 306. Each beveled edge has a thickness of 1/16 inch. In this embodiment, the glass plates are maintained in position by a pair of U-shaped brackets 308 and 310. Brackets 308 and 310 can be made from, for instance, ¾ inch finished plywood.
During testing, samples are placed over the beveled edges 304 and 306. Multiple images of the folded edges are then taken along the edge as shown at 312. Thirty (30) fields of view are examined on each folded edge to give a total of sixty (60) fields of view. Each view has “PR/EL” measured before and after removal of protruding fibers. “PR/EL” is perimeter per edge-length examined in each field-of-view. FIG. 6 illustrates the measurement taken. As shown, “PR” is the perimeter around the protruding fibers while “EL” is the length of the measured sample. The PR/EL valves are averaged and assembled into a histogram as an output page. This analysis is completed and the data is obtained using the QUANTIMET 970 Image Analysis System obtained from Leica Corp. of Deerfield, Ill. The QUIPS routine for performing this work, FUZZ10, is as follows:
Cambridge Instruments QUANTIMET 970 QUIPS/MX: VO8.02  USER:ROUTINE: FUZZIO DATE: 8-MAY-81    RUN: 0 SPECIMEN:NAME =FUZZBDOES =PR/EL ON TISSUES; GETS HISTOGRAMAUTH =B.E. KRESSNERDATE =10 DEC 97COND =MACROVIEWER; DCI 12×12; FOLLIES PINK FILTER; 3×3MASK 60 MM MICRO-NIKKO, F/4; 20 MM EXTENSION TUBES;2 PLATE (GLASS) FIXTURE MICRO-NIKKOR AT FULLEXTENSION FOR MAX MAG!ROTATE CAM 90 deg SO THAT IMAGE ON RIGHT SIDE!ALLOWS TYPICAL PHOTOEnter specimen identityScanner (No. 1 Chalnicon LV= 0.00 SENS= 2.36 PAUSE)Load Shading Corrector (pattern - FUZZ7)Calibrate User Specified (Cal Value − 9.709 microns per pixel)SUBRTN STANDARDTOTPREL: = 0.TOTFIELDS: = 0.PHOTO: = 0.MEAN: = 0.If PHOTO = 1, thenPause MessageWANT TYPICAL PHOTO (1 = YES; 0 = NO)?Input PHOTOEndifIf PHOTO = 1, thenPause MessageINPUT MEAN VALUE FOR PR/ELInput MEANEndifFor SAMPLE = 1 to 2If SAMPLE = 1, thenSTAGEX: = 36,000.STAGEY: = 144,000.Stage Move (STAGEX, STAGEY)Pause Messageplease position fixturePauseSTAGEX: = 120,000.STAGEY: = 144,000.Stage Move (STAGEX, STAGEY)Pause Messageplease focusDetect 2D (Darker than 54, Delin PAUSE)STAGEX: = 36,000.STAGEY: = 144,000.EndifIf SAMPLE = 2, thenSTAGEX: = 120,000.STAGEY: = 44,000.Stage Move (STAGEX, STAGEY)Pause Messageplease focusDetect 2D (Darker than 54, Delin)STAGEX: = 36,000.STAGEY: = 44,000.EndifStage Move (STAGEX, STAGEY)Stage Scan ( X Yscan originSTAGEXSTAGEYfield size6,410.078,000.0no of fields  30   1)For FIELDIf TOTFIELDS = 30, thenScanner (No. 1 Chalnicon AUTO-SENSITIVITY LV= 0.01)EndifLive Frame is Standard Image FrameImage Frame is Rectangle (X: 26, Y: 37, W: 823, H: 627)Scanner (No. 1 Chalnicon AUTO-SENSITIVITY LV= 0.01)Image Frame is Rectangle (X: 48, Y: 37, W: 803, H: 627)Detect 2D (Darker than 54, Delin)Amend (OPEN by 0)Measure field - Parameters into array FIELDBEFORPERI: = FIELD PERIMETERAmend (OPEN by 10)Measure field - Parameters into array FIELDAFTPERIM: = FIELD PERIMETERPROVEREL: = ((BEFORPERI − AFTPERIM) / (I.FRAME.H * CAL.CONST))TOTPREL: = TOTPREL + PROVERELTOTFIELDS: = TOTFIELDS + 1.If PHOTO = 1, thenIf PROVEREL > (0.95000 * MEAN) thenIf PROVEREL < (1.0500 * MEAN) thenScanner (No. 1 Chalnicon AUTO-SENSITIVITY LV= 0.01 PAUSE)Detect 2D (Darker than 53 and Lighter than 10, Delin PAUSE)EndifEndifEndifDistribute COUNT vs PROVEREL (Units MM/MM)  into GRAPH from 0.00 to 5.00 into 20 bins, differentialStage StepNext FIELDNextPrint “ ”Print “AVE PR-OVER-EL (UM/UM) =“, TOTPREL/TOTFIELDSPrint “ ”Print “TOTAL NUMBER OF FIELDS =“, TOTFIELDSPrint “ ”Print “FIELD HEIGHT (MM) = ”, I.FRAME.H * CAL.CONST / 1000Print “ ”Print “ ”Print Distribution (GRAPH, differential, bar chart, scale = 0.00)For LOOPCOUNT = 1 to 26Print “ ”NextEND OF PROGRAM
Papermaking fibers, as used herein, include all known cellulosic fibers or fiber mixes comprising cellulosic fibers. Fibers suitable for making the webs of this invention comprise any natural or synthetic cellulosic fibers including, but not limited to nonwoody fibers, such as cotton, abaca, kenaf, sabai grass, flax, esparto grass, straw, jute hemp, bagasse, milkweed floss fibers, and pineapple leaf fibers; and woody fibers such as those obtained from deciduous and coniferous trees, including softwood fibers, such as northern and southern softwood kraft fibers; hardwood fibers, such as eucalyptus, maple, birch, and aspen. Woody fibers can be prepared in high-yield or low-yield forms and can be pulped in any known method, including kraft, sulfite, high-yield pulping methods and other known pulping methods. Fibers prepared from organosolv pulping methods can also be used, including the fibers and methods disclosed in U.S. Pat. No. 4,793,898, issued Dec. 27, 1988, to Laamanen et al.; U.S. Pat. No. 4,594,130, issued Jun. 10, 1986, to Chang et al.; and U.S. Pat. No. 3,585,104. Useful fibers can also be produced by anthraquinone pulping, exemplified by U.S. Pat. No. 5,595,628, issued Jan. 21, 1997, to Gordon et al. A portion of the fibers, such as up to 50% or less by dry weight, or from about 5% to about 30% by dry weight, can be synthetic fibers such as rayon, polyolefin fibers, polyester fibers, bicomponent sheath-core fibers, multi-component binder fibers, and the like. An exemplary polyethylene fiber is Pulpex®, available from Hercules, Inc. (Wilmington, Del.). Any known bleaching method can be used. Synthetic cellulose fiber types include rayon in all its varieties and other fibers derived from viscose or chemically modified cellulose. Chemically treated natural cellulosic fibers can be used such as mercerized pulps, chemically stiffened or crosslinked fibers, or sulfonated fibers. For good mechanical properties in using papermaking fibers, it can be desirable that the fibers be relatively undamaged and largely unrefined or only lightly refined. While recycled fibers can be used, virgin fibers are generally useful for their mechanical properties and lack of contaminants. Mercerized fibers, regenerated cellulosic fibers, cellulose produced by microbes, rayon, and other cellulosic material or cellulosic derivatives can be used. Suitable papermaking fibers can also include recycled fibers, virgin fibers, or mixes thereof. In certain embodiments capable of high bulk and good compressive properties, the fibers can have a Canadian Standard Freeness of at least 200, more specifically at least 300, more specifically still at least 400, and most specifically at least 500.
Other papermaking fibers that can be used in the present invention include paper broke or recycled fibers and high yield fibers. High yield pulp fibers are those papermaking fibers produced by pulping processes providing a yield of about 65% or greater, more specifically about 75% or greater, and still more specifically about 75% to about 95%. Yield is the resulting amount of processed fibers expressed as a percentage of the initial wood mass. Such pulping processes include bleached chemithermomechanical pulp (BCTMP), chemithermomechanical pulp (CTMP), pressure/pressure thermomechanical pulp (PTMP), thermomechanical pulp (TMP), thermomechanical chemical pulp (TMCP), high yield sulfite pulps, and high yield Kraft pulps, all of which leave the resulting fibers with high levels of lignin. High yield fibers are well known for their stiffness in both dry and wet states relative to typical chemically pulped fibers.
Machine Direction Slope A or Cross-Machine Direction Slope A is a measure of the stiffness of a sheet and is also referred to as elastic modulus. The slope of a sample in the machine direction or the cross-machine direction is a measure of the slope of a stress-strain curve of a sheet taken during a test of tensile testing (see geometric mean tensile strength definition above) and is expressed in units of grams of force. In particular, the slope A is taken as the least squares fit of the data between stress values of 70 grams of force and 157 grams of force. The geometric mean slope A is then the square root of the quantity derived by multiplying the MD slope A times the CD slope A.
Machine Direction Coefficient of Friction and Cross-Machine Direction of Coefficient of Friction is obtained using the Kawabata Evaluation System (KES) test instrument KES model FB-4-S. The KES instrument is available from Kato Tech Co, Ltd. 26 Karato-Cho, Nishikugo, Minami-Ku Kyoto 6701-8447 Japan. The sample is placed on a specimen tray, and a holding frame is placed over the specimen. The machine direction measurement is taken first. Two probes, one to measure the coefficient of friction (reported as MIU) and one to measure the surface roughness (reported as SMD) are placed on the sample. The probe for measurement of surface roughness is made of a steel wire of diameter of 0.5 mm. The coefficient of friction is measured using a probe with 10 pieces of steel wires each 0.5 mm in diameter, and is designed to simulate the human finger. The sample is moved forward and backward underneath the two probes at a constant rate of 0.1 cm/sec. The measurement is taken for 2 cm over the surface. The distance or displacement of the probe is detected by a potentiometer. The coefficient of friction probe is detected by a force transducer. The vertical movements of the surface roughness probe are detected by a transducer. The displacement (distance) of the sample (L, cm) vs. the coefficient of friction (MIU-unitless) and surface roughness (SMD-μm) are plotted. The sample is then rotated 90 degrees and tested again to provide the cross machine direction measurements. The following settings were used:    Friction sensitivity=2×5    Roughness Sensitivity=2×5    Static Load=25 gWith the above settings, the raw numbers from the instrument are then multiplied by 0.2 to yield the final coefficient of friction results.
Kawabata Bending Stiffness was measured using the KES model FB-2, again available from the Kato Tech Company. To measure bending the sample is clamped in an upright position between two chucks and a 0.4 mm center adjustment plate is used (the size of the adjustment plate is dependent on the sample thickness). One of the chucks is stationary while the other rotates in a curvature between 2.5 cm−1 and −2.5 cm−1.
The movable chuck moves at a rate of 0.5 cm−1/sec. The amount of moment (grams force*cm/cm) taken to bend the material vs. the curvature is plotted. For all the materials tested, the following instrument settings were used:                Measurement mode=one cycle        Sensitivity=2×1        K Span Control=SET        Curvature=+/−2.5 cm−1         
The KES system algorithm computes the following bending characteristic values:                B=bending stiffness (grams force×cm2/cm)        2HB=bending hysteresis (grams force×cm/cm)        
Both MD and CD bending stiffness were tested for each sample, and the mean bending stiffness calculated by taking the arithmetic average of the MD and CD measurements. The mean bending stiffness is referred to herein as “Kawabata bending stiffness”.
Stiffness/GM A Slope is the Kawabata bending stiffness divided by the geometric mean (GM) slope A.
Compression Linearity is measured using the Kawabata Evaluation System KES model FB-3, again available from Kato Tech Company.
The instrument is designed to measure the compression properties of materials by compressing the sample between two plungers. To measure the compression properties, the top plunger is brought down on the sample at a constant rate until it reaches the maximum preset force. The displacement of the plunger is detected by a potentiometer. The amount of pressure taken to compress the sample (P, gf/cm2) vs. thickness (displacement) of the material (T, mm) is plotted on the computer screen. For all the materials in this study, the following instrument settings were used:                Sensitivity=2×5        Gear (speed)=1 mm/50 sec        Fm set=5.0        Stroke select=Max 5 mm        Compression area=2 cm2         Time lag=standard        Max compression force=50 gf         
The KES algorithm calculates the following compression characteristic values and displays them on a computer screen:                Compression Linearity (LC).        Compression Energy (WC)        Compression Resilience (RC).        Thickness value measured at the minimum pressure of 0.5 gf/cm2 (TO)        Thickness value measured at full compression pressure of 50 gf/cm2 (TM)The following formula was used to calculate the compression rate (EMC):        
      EMC    ⁢                  ⁢    %    =                    TO        -        TM            TO        ×    100  5 measurements were taken on each sample.
The compression linearity values are reported in the Examples.